All you need is a bottle of Simple Green, it will take it right off and won't hurt the finish. You can also use it to clean your engine, spray it on wait about 5 minutes and rinse. That stuff is amazing. It kept my diesel engines on my past truck so clean, you could eat off them.
2008 F-250 4X4 Lariat V-10
2002 Airstream Classic 30' w/SO #2074
2007 Kubota 900 RTV
1996 Ford Bronco
2007 Lincoln LT